This invention relates to the treatment of pyrophoric materials such as sponge iron.
Sponge iron is a product utilized in the steel making industry as a basic source for the production of steel. Generally speaking, sponge iron is produced by exposing hematite (Fe.sub.2 O.sub.3) iron ore in comminuted form to a reducing gas environment at temperatures somewhat below blast furnace temperatures. The production of sponge iron is the subject of a large number of patents, including the following U.S. Pat. Nos. 2,243,110; 2,793,946; 2,807,535; 2,900,247; 2,915,379; 3,128,174; 3,136,623; 3,136,624; 3,136,625; 3,375,098; 3,423,201; 3,684,486; 3,765,872; 3,770,421; 3,779,741; 3,816,102; 3,827,879; 3,890,142; and, 3,904,397. The final sponge iron product of practically all of the processes disclosed in these patents is in a particulate or pellet form.
Typically, the components of sponge iron are metallic iron, iron oxide, gangue and possibly carbon. Metallic iron is iron which has been totally reduced by the reducing gas environment. Gangue is the term used in the industry to refer to all non-ferrous material, except carbon contained in the ore. Gangue may include silica, alumnia, lime, magnesia, phosphorus, sulfer and possibly other materials. A deposit of carbon on the outside surface of the sponge iron particulate will be described in greater detail hereinafter. In all of the iron ore reduction processes just referred to, freshly produced sponge iron as found in the final vessel in the process is at a very high temperature, typically at 1500.degree. F. or even higher. The freshly produced sponge iron at high temperature must be moved from the final reactor to some type of storage location or be immediately utilized in a steel producing process. In the past, it was more typical that the freshly produced, high temperature sponge iron be used rather quickly in the production of steel. However, in the last few years, this situation has changed. There are more and more iron ore reducing plants being built in various parts of the world entirely removed from steel producing facilities. Therefore, it has become necessary that sponge iron be stored and even shipped long distances.
Freshly produced sponge iron at high temperatures is not a stable material. In fact, such sponge iron is pyrophoric and subject to degradation through oxidation by exposure to air or water.
Storage and shipment of sponge iron is not a new problem. But, the importance of pacifying the sponge iron has now reached great significance. Attempts to at least partially cool the sponge iron to a safe temperature are found in the prior art. It is known that freshly reduced sponge iron must be cooled down significantly. Some cooling has been incorporated into the reduction process. Generally, this initial cooling occurs while the just-reduced sponge iron is still in the reduction reactor. U.S. Pat. No. 3,904,397 of Celada and others discloses the utilization of cooled, spent reducing gas in such a cooling reactor. Other U.S. patents which refer generally to the utilization of a cooling step immediately after reduction include U.S. Pat. Nos. 3,765,872; 3,684,486; 3,136,625; 3,136,624; and, 3,136,623.
U.S. Pat. Nos. 3,816,102 of Celada et al. and 3,136,624 of Mader et al. disclose a process for coating or depositing a layer of carbon onto the hot sponge iron during the initial cooling of the just-reduced sponge iron. Carbon is deposited for the next step in the process, i.e., the electric furnace, which converts the iron to steel, and also the carbon present reacts with the remaining iron oxide to finish the reduction (FeO+C.fwdarw.CO+Fe). One of the results of the deposition of the carbon layer on the sponge iron is the formation of a protective shell against reoxidation of the hot sponge iron because iron combined with carbon such as Fe.sub.3 C is supposedly less sensitive to oxygenation than the reduced metallic sponge iron. "Storage and Transportation of HYL DRI Pellets" presented by Ing. Raul G. Quintero, Hylsa, S.A. and Mr. G. E. McCombs, Pullman Swindell, Third Direct Reduction Congress, Instituto Latinoamericano del Fierro y el Acero, Caracas, Venezuela, July, 1977. U.S. Pat. No. 3,423,201 of Celada et al. discloses a method for cooling sponge iron having such a carbon layer deposited thereon. In Celada U.S. Pat. No. 3,423,201, a second cooling step is initiated when the temperature of the reduced ferrous material in the cooling reactor has dropped below the value at which cracking of reducing gas (and thus depositing of carbon on the sponge iron particulate) occurs. The Celada U.S. Pat. No. 3,423,201 states that the sponge iron is cooled to a temperature "near room temperature."
Basically, all of the just-discussed patents disclose the cooling of the sponge iron while still in a reactor. In order to cool the sponge iron in a reactor, it is necessary for the cooling gas to flow through the pile of sponge iron. Typically, the cooling gas takes the paths of least resistance and therefore is not equally distributed among all the sponge iron particulate. Further, the cooling gas serves to deposit fines in particular locations out of the flow paths of direct cooling gas flow so that hot spots of fines are formed. Such fines may also clog flow paths through the particulate and thus prevent cooling.
If the sponge iron is dumped from the reactor with certain portions at dangerously high temperatures, the likelihood of a significant portion of the entire batch or pile of sponge iron being eventually re-oxidized is high. This re-oxidation may not occur until the sponge iron is already on board a ship and in the middle of an ocean. The dangers to personnel of this type of fire, in addition to the economic loss, are considerable.
Another proposed solution to this problem has been suggested by the Midrex Corporation. Midrex Corporation has made public a chemical treating process sold under the trademark CHEMAIRE. The CHEMAIRE process is a combination of chemical treatment and air passivation to inhibit rusting and re-oxidation. "Direct From Midrex," Vol. 3, No. 2 brochure. Disadvantages of this type of system are several. First of all, the complete distribution of the chemical upon the particulate sponge iron is very unlikely. Secondly, the addition of the chemicals may or may not have any effect upon subsequent use of the sponge iron in the production of steel.
In summary, it was long ago recognized that sponge iron must be reduced below dangerous temperature levels in order to prevent re-oxidation. The main or most common method that has been used as disclosed in U.S. patents has been to cool the sponge iron while in the final reactor. Industrial practice has proven that cooling of the sponge iron in the final reactor has not been totally satisfactory. One possible solution to the problem as shown in the prior patents has been to dispose a layer of carbon upon the sponge iron during the cooling process. However, it has been found in practice that the carbon layer does not eliminate the problem since the core of the particulate sponge iron pellets may remain at too high a temperature and eventually re-oxidize upon exposure to air or water. Finally, a chemical treatment has been proposed, one disadvantage to such treatment being a likelihood of inadequate distribution of the chemical upon the particulate sponge iron.